The head of the UN nuclear watchdog has warned that Japan's battle to regain control at the failing Fukushima power plant is a race against time, as the crisis enters a second week.

Yukiya Amano urged the prime minister, Naoto Kan, to provide more detailed information. Japanese media reported that Kan pledged to do so.

Earlier, Amano told reporters: "This is not something that just Japan should deal with and people of the entire world should co-operate with Japan and the people in the disaster areas."

The International Atomic Energy Agency chief said staff would monitor radiation in Tokyo and then closer to the plant, Kyodo news agency reported.

Kan later told a press conference: "Everything has been disclosed to the public. We have shared what we know with the international community about the current situation. It is still very grave.

"In the not so distant future, it will be controlled and we will be able to emerge from the crisis. We are making every effort towards that end."

He said police, firefighters, the self defence forces and workers were putting their lives on the line to resolve the situation.

Japan's nuclear safety agency said the problems at the Fukushima No 1 power station were at level five – "accident with wider consequences" – on an international scale used to assess such incidents. That puts the damaged caused by last Friday's natural disaster at the same level of gravity as the Three Mile Island nuclear accident in the US in 1979.

Events have escalated since the agency rated the Fukushima situation as level four, having "local consequences", last Saturday. The French nuclear watchdog has argued it constitutes a "serious accident", which would be level six. The Chernobyl disaster in 1986 rated seven, the highest point on the scale.

Fire trucks doused a reactor at the facility with tonnes of water on Friday in a renewed attempt to cool reactor 3.

It is thought spent fuel rods were exposed as water levels dropped in a storage pool, leading to significant increases in radiation levels.

There is also concern about water levels in the pools at reactors 1 and 4; the pools at reactors 5 and 6 are also thought to be warming.

Radiation readings taken 1km west of reactor 2 offered some hope, dropping from 351.4 microsieverts per hour just after midnight to 265 microsieverts per hour at 11am. But there have been enormous variations in readings at different parts of the plant and within short spaces of times.

Hidehiko Nishiyama, a spokesman for the Japanese nuclear agency, said steam or smoke was seen on Friday morning at reactor 2, where the containment vessel is damaged. He said the authorities could yet bury the reactors in sand and concrete, as happened at Chernobyl.

Nishiyama said the priority was adding water to the spent fuel pools. Asked about the "Chernobyl solution", he replied: "That solution is in the back of our minds, but we are focused on cooling the reactors down."

However, the head of the US nuclear regulatory commission, Gregory Jaczko, warned on Thursday that the units may not cool down for weeks. Jaczko said the situation "continues to be very dramatic", adding: "I really don't want to speculate on where this could go."

The plant's operators say workers are attempting to restore power to the cooling systems of two reactors by the end of the day and two more by Sunday. But there are fears that the systems themselves may have been damaged.

In the US, President Barack Obama said the situation posed a substantial risk to residents near the Fukushima plant. The US has advised its citizens to evacuate or take shelter if within 80km (50 miles) of the plant, a recommendation adopted by Britain and Canada.

Tokyo has told everyone living within 20km to evacuate, and advised people between 20km and 30km away to stay indoors. The ministry of health said it had ordered local governments to test the radioactivity levels of domestically produced food.